PTOs are used on many types of vehicles, including on agricultural vehicles such as tractors, to provide power for equipment or implements, such as, for agricultural purposes, combines, mowers, balers, forage harvesters and spreaders.
Modern tractors commonly have horsepower ratings in excess of 100 horsepower. However, the shaft sizes for PTOs have not changed due to the need to maintain compatibility with older equipment and maintain the standardization for PTOs. Thus, the torque output of PTOs for many modern tractors is no longer limited by the tractor horsepower. Rather, the torque output is limited by the strength of the PTO shaft and the failure thereof. In addition to causing PTO shaft failures, the torque produced by the high horsepower tractors can accelerate equipment attached to the respective PTO at a rate which can damage the equipment.
In view of the problems associated with the control of PTO shafts in high horsepower tractors, it was found desirable to provide a PTO clutch control system for protecting PTO shafts from catastrophic failure and for providing PTO shaft accelerations at rates which protect the shafts and attached equipment during clutch engagement.
Typical of such a system is the system of U.S. Pat. No. 5,494,142, which discloses a PTO control system for vehicles, such as farm tractors including a power take-off (PTO) shaft, for supplying rotational motion to an implement of the type which may be stationary or towed by the tractor. Power is transferred to the PTO shaft by a clutch including an input shaft coupled to a power source and an output shaft coupled to the PTO shaft. The clutch transmits a maximum torque between the input and output shafts in response to a maximum clutch pressure and transmits a variable torque between the input and output shafts in response to a given clutch engagement pressure that is less than the maximum clutch engagement pressure. Typically, a generally linear, gentle ramping up of current/pressure is employed to achieve smooth engagement.
The control system includes a first transducer disposed to generate an input signal representative of the rotational speed of the input shaft, a second transducer disposed to generate an output signal representative of the rotational speed of the output shaft, and a control circuit. The control circuit is coupled to the clutch control, the first transducer, and the second transducer.
While such a control system has been of great value and effectiveness, it and other control systems have continued to experience difficulties when attempts are made to drive PTO under extreme conditions. With such systems, no differentiation was made with respect to the loads applied, be they very light or very heavy. With a light applied load, initial PTO shaft movement could occur at a relatively early time and the full shaft speed would be reached before a modulation is effectively executed. With a heavy load, however, initial PTO shaft movement would not occur until a later time, leaving very little time for modulation.
The strategy of employing a generally linear, gentle ramping up of current/pressure to achieve smooth engagement, while effective and beneficial in many instances, has nevertheless been found to be not as effective as would be desirable in effecting initial movement of extreme loads and smooth engagement. In various instances, the difficulty in initiating movement could result in either abandonment by the system of the engagement or by a sudden and abrupt engagement, which, in severe cases, could lead to breakage of the shaft or unsafe operation.